Manganese oxide materials (as used herein, manganese oxide materials are denoted as MnO2, but refers to all species of manganese oxide compounds including, but not limited to: MnO2, MnOx, MnOx. yH2O, or MnOxHy, where x=1.5 to 2.5 and y=0.5 to 2, doped manganese oxides, and AMnO2 where A= alkali-metal or alkaline-earth cations) have long been explored for use as cathode materials for batteries, and manganese oxide is used in the familiar 1.5-volt commercial Zn/MnO2 alkaline cells (Chabre et al.,1995). More recently, manganese oxide materials have been studied as intercalation hosts for lithium batteries (Thackeray, 1997), which can provide voltages of 3 to 5 volts. The intense interest in manganese oxide as a battery material is because of its low cost and low toxicity relative to other high performance metal oxide battery materials such as NiO2, CoO2, and V2O5.
The discharge processes at MnO2 are accomplished by the intercalation of either protons or lithium cations into the MnO2 structure. This process is accompanied by a simultaneous reduction of the manganese sites to maintain charge balance:                MnO2+xH++xe→HxMnO2         MnO2+xLi++xe→LixMnO2         
The protons or lithium ions intercalated into the MnO2 structure are supplied by the electrolyte at the electrode/electrolyte interface. Charge storage is further facilitated by diffusion of protons or lithium cations though the bulk of the structure.
Although MnO2 materials are relatively inexpensive and are currently used in commercial batteries, some problems remain, particularly with respect to lithium battery applications. For example, electrodes made from manganese oxide spinels have poor conductivity and require the addition of conductive fillers such as carbon to enhance conductivity. However, adding such fillers reduces the energy density of the electrode. Moreover, recharging the cells requires applying a voltage which exceeds the discharge terminal voltage of the cell. The result, for cells having such manganese-oxide spinel cathodes, is that it takes at least 4.1 volts (and preferably more) to de-intercalate lithium from the electrode during charging of the cell. Above about 4.5 volts, however, the solvents used as the electrolyte oxidize and decompose. It is, therefore, necessary to control the charging voltage of these cells below the decomposition potential of the solvent in order to prevent its degradation.
In addition, due to the crystalline structure of spinel manganese oxide, the reversible capacity and cycle life of spinel-based cathodes are sensitive to overcharge and over-discharge. Discharge of the manganese oxide spinel cells must be cut off when the terminal voltage falls to about 3.4 volts (thus limiting the capacity of the material, which typically peaks at about 140 mAh/g). Below about 3.4 volts, the spinel form of the manganese oxide undergoes structural transformation when additional lithium is inserted into LiMn2O4 and it converts to the orthorhombic form which has very poor cycle ability, and is very unstable, causing the formation of other manganese oxides which are not electrochemically active.
Moreover, insertion of more than one lithium ion per molecule into spinel manganese oxide results in cation mixing between octahedral and tetrahedral sites, which leads to continuous capacity decay. To avoid these problems, the cell voltage must be controlled electronically during the operation of the cell. Such control is-very difficult to manage when a number of large lithium cells are coupled together in series. Spinel-type manganese oxide electrodes typically have internal surface areas less than about 40 m2/g, which limits the rate at which they can be discharged.
Charge/discharge rates and the capacity achieved at those rates are in part determined by the transport of protons or lithium cations through the MnO2 structure. Sol-gel-derived manganese oxides are typically microporous, based on the tendency of MnO2 to form tunnel or layered structures. Intercalating cations must also be transported through the micropores. The small pore size can limit the accessability of electrolyte to the MnO2 interface, particularly for the large (relative to the proton) lithium cations.
Manganese oxide can be produced in a variety of forms, the most common form for battery materials being electrolytic manganese dioxide (EMD) (Chabre et al., 1995). EMD is prepared by anodic electrodeposition from manganese (II) salts. Although EMD has been used in alkaline batteries for many years, recent investigations have shown that it is not optimal for lithium battery applications (Bach et al., 1992). Some efforts have been made to improve the surface area and porosity of EMD (Kurimoto et al., 1995).
Manganese oxides have also been prepared by a variety of sol-gel approaches (Manthiram et al., 1998). Sol-gel chemistry provides a flexible, low temperature process for preparing metal oxides. Another advantage of sol-gel chemistry is that dopant ions can be mixed uniformly in the manganese sol to improve the electrochemical and structural properties of the manganese oxide. Under the appropriate reaction conditions, the sol-to-gel transition can occur so that the metal oxide sol forms a highly porous three-dimensional network. Removal of the pore fluid exclusively by evaporation typically collapses the porous structure due to the large capillary forces exerted on the gel structure at the liquid-gas interface.
If the pore fluid is removed under conditions in which capillary forces are low or extremely low, the inherent mesoporosity and high surface area of the initial gel can be retained. Aerogels are prepared by taking the pore fluid supercritical, wherein there is no longer a liquid-gas interface (Huesing et al., 1998). When supercritical CO2 drying is preferred, the pore liquid of the wet gel is replaced with liquid CO2, which is then taken supercritical. Aerogels of V2O5 have been prepared that exhibit both high surface areas and high porosities (Salloux et al., 1995; Le et al., 1996; Le et al., 1995).
An alternative to supercritical drying is replacing the pore fluid with a low surface tension liquid, such as an alkane, and evaporating at ambient conditions. Ambient pressure synthesis of V2O5 (Coustier et al., 1998; Harreld et al., 1998) and MoO3 (Harreld et al., 1998) gels (now denoted as ambigels) have been accomplished. Ambigels exhibit a porosity between that of xerogels and aerogels. V2O5 aerogels and ambigels have both shown improved lithium capacities relative to xerogels derived from the same sol-gel chemistry (Dong et al., 2000).
Le et al., in U.S. Pat. No: 5,674,642, describe xerogels, cryogels, and aerogels of V2O5 synthesized from sols and gels.
Lynch, in U.S. Pat. No. 3,977,993, discloses preparing metal oxide aerogels by introducing an aqueous slurry of a hydrogel into an organic solvent such as ethanol until substantially all of the water in the hydrogel is displaced by the organic solvent. The organic solvent is then treated to render it rigorously anhydrous. The organic solvent is removed therefrom by heating the mixture to above the critical point and releasing the organic solvent therefrom at a pressure at least equal to the critical pressure of the organic solvent.
Tillotson et al., in U.S. Pat. Nos. 5,275,796 and 5,409,683; describe a two-step hydrolysis condensation method to form metal oxide aerogels. A high purity metal alkoxide is reacted with water, alcohol solvent, and an additive to form a partially condensed metal intermediate. All solvent and reaction-generated alcohol is removed, and the intermediate is diluted with a nonalcoholic solvent. Aerogels are formed by reacting the intermediate with water, nonalcoholic solvent, and a catalyst, and directly extracting the nonalcoholic solvent.
Anderson et al., in U.S. Pat. No. 5,227,342, disclose making porous ceramic materials with controlled porosity by manipulating the sol used to make the material by gradually removing protons from the metal oxide sol to a predefined threshold.
Hupe et al., in U.S. Pat. No. 4,894,357, disclose that the structural and/or surface characteristics of metal oxides can be adjusted by dehydrating a water-containing oxide gel under supercritical conditions by extracting the water with an extraction agent such as carbon dioxide at a pressure above the critical pressure of the extraction agent.
Dasgupta et al., 5,601,952, disclose preparing lithium-manganese oxide compounds which can be used in a non-aqueous rechargeable lithium battery. A gel of lithium manganese oxide is prepared in a water-miscible organic solvent such as an alcohol. The gel is dried and, depending upon the method of liquid removal a xerogel, aerogel, sonogel, or cryogel is obtained.
Passerini et al., (1999) describe the preparation of MnO2 Xerogels and ambigels (hexanogels in their terminology).
However, to date there has been no method to obtain high surface area, highly mesoporous MnO2 with a controlled, continuously intertwined solid-pore architecture on the nanoscale.